1. Field of the Invention
The present invention relates to amplifier klystrons with wide instantaneous passbands. It can be applied to single-beam klystrons as well as to multiple-beam klystrons. The instantaneous passband is the frequency band in which the gain of the tube is greater than a limit, for example one dB below its maximum value.
A single-beam klystron is a microwave tube with velocity modulation of an electron beam. Its principle is based on the interaction between a longitudinal electron beam and electromagnetic fields induced in resonant cavities. The electrical component of the electromagnetic field is parallel to the axis of the electron beam. A focusing device surrounds the cavities. This device prevents the electron beam from diverging. The magnetic field created by this device is parallel to the axis of the electron beam.
The cavities, which are generally four or five in number, are placed one after the other. They are separated by drift tubes which are small-diameter tubes. The interval between two drift tubes is an interaction space. The electron beam, formed in a gun, successively crosses the resonant cavities and the drift tubes. A microwave to be amplified is introduced into the first cavity or input cavity. The last cavity or output cavity is connected to an output device. The electron beam acquires a velocity modulation in entering the first cavity. This velocity modulation is converted into a density modulation in the drift tube placed downline, that is further along the direction of movement of the electron beam, e.g. the second cavity is downline of the first cavity, from the first cavity, and this enables the second cavity to be excited.
The electrons come together in increasingly dense packets. These packets are obtained by the action of all the cavities except the last one, and by the passive effect of the drift tubes. The cavities modulate the velocity of the electron beam. In the drift tubes, the fast electrons catch up with the slower electrons.
In the last cavity, the highly modulated electron beam yields its energy, by being stopped, to the electromagnetic field of this cavity, and this energy gets propagated up to the output device.
A multiple-beam klystron has one or more guns that produce several parallel, longitudinal electron beams. These electron beams go through a succession of cavities. A cavity is crossed by all the beams. Two successive cavities are connected by as many drift tubes as there are electron beams. The working of a multiple-beam klystron is comparable to that of a single-beam klystron.
If the cavities of a klystron are all tuned to the same resonance frequency, then the instantaneous passband, measured at -1 dB, will be small, in the range of 1% for example.
However, there are amplifier klystrons with wider instantaneous passbands, with values of the order of several per cent, and even of up to 10%.
To obtain such a result, the method employed is that of stagger-tuned amplifiers: in this method, each cavity is tuned to a frequency different from that of its neighbors.
Almost all the tuning frequencies are distributed in the passband that the klystron should have.
However, the fine tuning of a wideband klystron with stagger tuning is a complex process. For, the curve of the gain as a function of the frequency of a cavity, associated with its two drift tubes, resembles that of a parallel R, L, C circuit, close to its resonance frequency with a maximum, but it also has a minimum for a certain frequency which is generally higher than the resonance frequency.
It is seen that, if the sum of the lengths of the two drift tubes adjacent to the cavity is substantially equal to 180 plasma degrees, the minimum gain is pushed towards infinity.
The length of drift tubes is expressed, in a standard way, in terms of the reduced plasma angle, i.e. in terms of "plasma degrees" plasma indicating the interior of the electron beam. The length of a drift tube L in plasma degrees is given by: EQU L(360.times.d)/1.sub.q
where 1.sub.q is the wavelength of plasma and d is the physical distance between the centers of two interaction spaces placed on either side of the drift tube, in the corresponding cavities.
Furthermore, in klystrons with more than three cavities, the response of a cavity, located in the central part of the tube, is affected by what has happened in the previous cavities. The electron beam has been modulated in the previous cavities, and the closer the beam gets to the last cavity, the more modulated it is. The electron packets are increasingly dense, the phenomena are no longer linear and the modulations are no longer simply added on to each other. It is necessary to take account of the space charge, namely the mutual repulsion between electrons.
An instantaneous wideband klystron, with four cavities, has its input cavity and its output cavity tuned to the central frequency Fo of the passband that the klystron should have. The second cavity is generally tuned to a frequency lower than the central frequency Fo while the third cavity is tuned to a frequency higher than the central frequency Fo. A known method of obtaining as wide a passband as possible is to ensure that the second cavity and the third cavity each have adjacent drift tubes with a length such that their sum is substantially equal to 180 plasma degrees.
The total length of the drift tubes of the klystron is then substantially equal to 270 plasma degrees.
If the klystron has more than four cavities, it is common practice to limit the total length of its drift tubes to about 270 plasma degrees. This value of 270 plasma degrees does not have to be met very strictly, and may furthermore be modified as a function of other characteristics.
It can be seen that it is possible to add on cavities to widen the passband of the klystron and these cavities are preferably tuned to frequencies higher than the central frequency Fo. It can also be easily seen that the cavities added on after the sixth and seventh cavities no longer make any great contribution to increasing the passband of the klystron. Furthermore, as a result of the limitation of the value of 270 plasma degrees, the added cavities are extremely close to one another. It may even be that they have to overlap one another, which cannot be achieved in practice. In any case, it becomes difficult to build the tube. The widest instantaneous passband obtained generally does not go beyond 10%.